### Bias

**Bias is error of** Machine learning **model**. It is inability for a Machine learning algorithm to capture true relationship.

An **overly simplified model** (straight line) **has high Bias error**.

**Complex model** (squiggly line)** has low Bias error**.

### Variance

**Variance is difference in error of** Machine learning **model between training data and test data**.

**High Variance is when error difference **between Train dataset and Test dataset **is high.**

**Low Variance is when error difference **between Train dataset and Test dataset **is low.**

### Underfitting and Overfitting

**When model has High Bias **then we say that our **model is Underfitting** the training data, **because the model performs poorly on the training data**. This is because the model is unable to capture the relationship between the input examples (often called X) and the output values (often called Y).

**When model has Low Bias and High Variance** then we say that **our model is Overfitting** training data, **because the model performs well on the training data but does not perform well on the evaluation data**. This is because the model is memorizing the data it has seen and is unable to generalize to unseen examples.

### Bias and Variance cases

When looking **on** algorithm **error on training set and **algorithm** error on test set you can diagnose whether it has problems of high bias or high variance or maybe both or maybe neither. With** Bias and Variance **diagnosis you can **try different things to** make better Machine learning model.**

#### Case High Bias

Train set error – 15%

Test set error – 16%

This case has High Bias problem. **To fix this this you could do:**

**make bigger network**(more hidden layers and more neurons)**train longer****choose different neural network architecture**

#### Case High Variance

Train set error – 1%

Test set error – 11%

This case has High Variance problem. **To fix this this you can do:**

**Get more data****Regularization**

#### Case High Bias and High Variance

Train set error – 15%

Test set error – 30%

This case has High Bias and High Variance problem. **To fix this you could make trade-off between High Bias and High Variance **by using above mentioned techniques for decreasing High Bias and High Variance.

#### Case Low Bias and Low Variance

Train set error – 0.5%

Test set error – 1%

This case has Low Bias and Low Variance problem. **This is what a good model should look like** and for this case no actions are required.

Thanks for reading this post.

### References

- Coursera. 2020.
*Bias / Variance – Practical Aspects Of Deep Learning | Coursera*. [online] Available at: <https://www.coursera.org/learn/deep-neural-network/lecture/ZhclI/bias-variance> [Accessed 27 May 2020]. - Coursera. 2020.
*Basic Recipe For Machine Learning – Practical Aspects Of Deep Learning | Coursera*. [online] Available at: <https://www.coursera.org/learn/deep-neural-network/lecture/ZBkx4/basic-recipe-for-machine-learning> [Accessed 28 May 2020]. - 2020.
*Machine Learning Fundamentals: Bias And Variance*. [online] Available at: <https://www.youtube.com/watch?v=EuBBz3bI-aA> [Accessed 27 May 2020]. - Docs.aws.amazon.com. 2020.
*Model Fit: Underfitting Vs. Overfitting – Amazon Machine Learning*. [online] Available at: <https://docs.aws.amazon.com/machine-learning/latest/dg/model-fit-underfitting-vs-overfitting.html> [Accessed 28 May 2020].